Acid corrosion may cause the deterioration of the fire resistance of the intumescent fire retardant coatings. This paper reported the results of an experimental study of degradation in fire protection performance of intumescent coating for steel elements after different times of immersion in the hydrochloric acid solution. The morphology, the equivalent thermal resistance, and the changes of the elements and groups contained on the surface of the coatings were investigated. The corrosion mechanism was as follows: when the coating was immersed in the acid solution, the ammonium polyphosphate (APP), pentaerythritol (PER), and melamine (MEL) underwent the process of acid catalyzed hydrolysis, dissolution, and chemical reaction, which led the change of the content ratio of the compositions and caused the deterioration of the coating fire resistance. A prediction method of the remaining fire resistance property for the acid corrosion coatings was proposed, by comprehensively considering the physical and chemical characteristic changing trends of the corroded coating.